Laser scanning apparatus and method

ABSTRACT

The disclosed embodiments include an apparatus and method of using a laser to scan the ground or a target from an airborne or ground-based platform. In certain embodiments, the apparatus and method produces a 3-D elevation model of the terrain. In some embodiments, the apparatus includes a pulsed laser, a receiver to detect and amplify the pulse after being reflected by objects on the ground (or the ground itself), and electronics which measures the time of flight of the optical pulse from which the slant range to the target is calculated. Technical advantages of the disclosed embodiments include avoiding blind zones to ensure that no laser shots are wasted. In certain embodiments for airborne applications, the apparatus may also be configured to maintain a constant swath width or constant spot spacing independent of aircraft altitude or ground terrain elevation.

CROSS-REFERENCE TO RELATED APP ACATIONS

This application is a divisional of co-pending U.S. Non-Provisionalpatent application Ser. No. 14/639,320, filed Mar. 5, 2015, and entitled“IMPROVED LASER SCANNING APPARATUS AND METHOD” the disclosure of whichis expressly incorporated by reference herein in its entirety.

TECHNICAL FIELD

Embodiments of the invention are directed, in general, to providing animproved apparatus and method for 3D measurement of land topography froman airborne or ground-based platform and, in particular, one thateliminates the potential loss of data that can be caused by blind zoneswhich can occur in existing laser terrain mapping systems.

BACKGROUND

Airborne Laser Terrain Mapping (ALTM) systems use a Time-of-Flight (TOF)LiDAR to measure the distance from a system mounted in an aircraft, tothe ground beneath the aircraft. A short pulse of visible or infra-redlight is emitted by a light source such as a laser, and directed towardsa target. The light pulse propagates to the target and a fraction isreflected and travels back to the LiDAR system where it is detected by ahigh-sped optical detector such as an avalanche photodiode, whichconverts the light pulse to an electrical signal which is thenamplified. By measuring the time interval from the instant the lightpulse was emitted to when the return signal was received, the distanceto the target can be calculated using the accurately-known speed ofpropagation of the light pulse. The TOF can be measured by an electronicsubsystem such as a Time Interval Meter, by digitizing the echo receivedand analyzing the waveform, or other means.

When the laser is fired, there is a very brief period when the detectormight see some scattered light. This could be caused by reflections frominternal optical components, a window at the output of the system, awindow in the aircraft through which the system operates orbackscattering from the first few meters of the air below the aircraft.If the echo from a previously-emitted laser pulse were to arrive at thedetector during this brief period, it would not be distinguishable fromthe scattered light pulse and if the scattered light produced a signalof much higher amplitude than the return pulse from the target, it wouldswamp the echo and render the system blinded for a period of time. Thepulse from the unwanted scattered light causes a blind zone during whichthe system is not able to respond to the return signal and measure theTOF. Consequently no range data can be computed and essentially thelaser shot is wasted. Currently, all existing airborne laser mappingsystems have this limitation.

For an ALTM operating at a high pulse repetition frequency (PRF), therange to the target could be such that the TOF is many times the timeinterval between two successive firings of the laser. Firing the laserbefore the return pulse from the target is received results in more thanone pulse in the air at the same time. If for example the target rangeand laser PRF were such that there were five pulses in the air at thesame time, there could be five blind zones which would significantlyincrease the possibility of the echo being masked and reducing thepossibility of obtaining a valid range measurement. Planning the flightaltitude to minimize the impact of blind zones is virtually impossibleat high laser PRFs because the TOF changes with aircraft height aboveground, the scanner excursion angle, the aircraft roll, pitch or headingas well as the topography of the terrain itself.

SUMMARY OF THE INVENTION

In general, the disclosed embodiments elate to the challenge of dealingwith multiple pulses of light that are in transit to and from the targetat the same time. The goal is to prevent the outgoing and incomingpulses from being incident on the detector at the same instant, whichwould make the system “blind” to incoming return pulses.

Accordingly, the disclosed embodiments include a system or apparatus andone or more methods for eliminating the negative effect of blind zonesand enables operation of the system at high laser PRFs without loss ofdata. Consequently, the disclosed embodiments have the potential tocollect valid data at the full laser PRF.

In addition to blind zones caused by the unwanted scattered lightdescribed above, under certain atmospheric conditions, blind zones canalso be caused when the system detects backscattered light from thefirst few meters of the air below the aircraft. For instance, the returnsignal could be from humid air 10 m below the aircraft, or it could be aground return pulse from ten laser shots ago, that is finally arrivingat the detector. Thus, certain embodiments disclosed herein areconfigured to significantly reduce the probability of this occurring andextending the blind zone. For example, certain embodiments may includespecial optical elements and a scanner that prevents the detection ofunwanted return pulses from the first few meters of the atmosphere.Without the optical elements, the system would be swamped by non-desiredsignals. In one embodiment, the disclosed system will reduce oreliminate blind zones caused by backscattered light within 20-50 m belowthe aircraft.

As will be further described, in certain embodiments, advantages of thedisclosed embodiments are accomplished using an electronic circuit thatcaptures the time interval between the emitted light pulse andsubsequent optical signals that are incident on the detector above acertain threshold. These signals could be a result of a) theback-scattered light from the outgoing light pulse by optical surfaces,b) the back-scattered light from the outgoing light pulse by theatmosphere close to the aircraft, or c) a return pulse from the intendedtarget. In certain situations, no return pulses are received (e.g.,altitude too high, hazy atmosphere, target reflectance too low);whereas, at other times, multiple return pulses may be received from asingle laser pulse (from a wire, or top of a tree, or branches beneath,or from the ground). In accordance with the disclosed embodiments, everyone of these detected events results in a TOF measurement. In certainembodiments, this is achieved through a hardware solution.

In one embodiment, the detected signals are monitored in real time andthe resultant range to the ground is computed. An algorithm identifiesand differentiates outgoing signals from return pulses. A signalresulting from the scattering of the output pulse by the internaloptical components or windows is identified as such by virtue of thetime at which it occurs. Said time being synchronous with the time ofemission of the laser pulse. For each return pulse, as will be describedbelow, an algorithm checks for the potential of an outgoing and incomingsignal to be incident on the detector at the same instant, theoccurrence of which is referred herein as a collision. The time span inwhich this collision may occur is referred to as a blind zone. If it ispredicted that this will occur, the system makes a very tiny adjustment(e.g., a fraction of a millionth of a second) to the time of sending outthe next laser shot (the outgoing signal) in order to prevent thecollision from occurring, and thus eliminating the blind zone. Ineffect, the firing of the laser is either delayed or advanced so thatthe outgoing laser pulse is placed into a time period where the returnsignal is not expected to be incident on the detector. The results arecontinuously monitored and adjusted as required. In one embodiment, thisis done shot-by-shot at the laser firing rate which can be over half amillion shots per second.

Another advantage of the disclosed embodiments includes an apparatus andmethod for maintaining a constant swath width and point distribution onthe ground, independent of the altitude of the aircraft or the elevationof the ground terrain.

As an example embodiment, the disclosed apparatus may include aprocessor for executing computer-executable instructions and acomputer-readable storage media for storing the computer-executableinstructions. These instructions when executed by the processor enablethe apparatus to perform features including dynamically monitoring thetime of flight (TOF) of laser light pulses transmitted and received bythe laser scanning apparatus; determining whether there is a potentialfor the outgoing laser light pulse and an incoming signal to be detectedwithin a few nanoseconds of each other; and adjusting a pulse repetitionfrequency (PRF) in response to a determination that the potentialsimultaneous (within a few nanoseconds) detection of the outgoing laserlight and the incoming signal is likely to occur. Other instructions mayinclude dynamically adjusting the scanner parameters to keep a spotdensity relatively constant as at least one of an aircraft flying heightand ground terrain elevation changes during a survey mission formaintaining a constant swath width using a laser scanning apparatus.

One example of the various embodiments disclosed herein include a systemthat is adapted to be mounted on an airborne platform for themeasurement of topographic elevations, wherein the system includes apuled laser for generating a pulse of light; a primary mirror adapted tooscillate back and forth in at least one axis so as to direct the laserlight to the ground in a pattern and further adapted to receivereflections of the laser light from the ground and direct thereflections of the laser light to a secondary mirror, the secondarymirror adapted to reposition and maintain the received laser light ontothe center of a detector, the detector configured to produce anelectrical signal that is amplified by a receiver; a time interval meterconfigured to determine the time of flight of the received laser light;and control electronics configured to determine the measurement oftopographic elevations beneath the airborne platform using the time offlight of the received laser light. In one embodiment, the secondarymirror is located between the primary mirror and a re-imaging module.

Other embodiments and advantages of the disclosed systems and methodsare further described in detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the description provided herein andthe advantages thereof, reference is not made to the brief descriptionsbelow, taken in connection with the accompanying drawings and detaileddescription, wherein like reference numerals represent like parts.

FIG. 1 is a block diagram illustrating a system in accordance with oneembodiment.

FIG. 2 is a schematic view of the system in operation.

FIG. 3 is an example of a timing diagram illustrating the case wherethere is only one pulse in transit at any given time in accordance withthe disclosed embodiments.

FIG. 4 is an example of a timing diagram illustrating the case wherethere are two optical pulses in transit at the same time in accordancewith the disclosed embodiments.

FIG. 5 is a flow chart illustrating a method of changing the laserfiring time to avoid potential blind in accordance with one embodiment.

DETAILED DESCRIPTION

The invention summarized above may be better understood by referring tothe following description, which should be read in conjunction with theaccompanying drawings. This description of an embodiment, set out belowto enable one to build and use an implementation of the invention, isnot intended to limit the invention, but to serve as a particularexample thereof. For instance, although certain embodiments describedherein focuses on an airborne application of the invention, otherembodiments of the invention can include ground-based laser scanningapplications, using either mobile or static platforms.

Those skilled in the art should appreciate that they may readily use theconception and specific embodiments disclosed as a basis for modifyingor designing other methods and systems for carrying out the samepurposes of the present invention. Those skilled in the art should alsorealize that such equivalent assemblies do not depart from the spiritand scope of the invention in its broadest form.

In addition, in the description which follows the drawing figures arenot necessarily to scale and certain features may be shown ingeneralized or schematic form in the interest of clarity and concisenessor for informational purposes, and do not limit the scope of the claims.

Additionally, although specific terms are employed herein, they are usedin a generic and descriptive sense only and not for purposes oflimitation. For instance, the term computer, as used herein, is intendedto include the necessary electronic components such as, but not limitedto, memory and processing components that are configured to enable theexecution of programmed instructions.

As described herein, the disclosed embodiments include an improved laserscanning apparatus and method that are configured to prevent loss ofdata caused by blind zones. For example, in one embodiment, based on theTOF measurement from the previous laser shot, or a sequence of previouslaser shots, the apparatus includes a data acquisition computer or otherelectronics that predicts whether the return signal from the next lasershot is likely to fall into a blind zone if operation is continued atthe current laser PRF. If the apparatus determines that the returnsignal from the next laser shot is likely to fall into a blind zone ifoperation is continued at the current laser PRF, the apparatus isconfigured to either advance or delay the laser firing time to ensurethat the return signal will be clear of the blind zone. In someembodiments, the adjustment process is performed on a shot by shot basiswhile maintaining the average data collection rate that was planned forthe mission.

In the embodiment, to reduce the potential for backscattered atmosphericreturns, the apparatus is configured to reduce the laser power to nomore than what is needed for reliably obtaining range measurements atthe chosen flying height. The apparatus is also configured to control asmall mirror which controls the receiver pointing direction. Oneadvantage of this additional mirror configuration and control process isthat it reduces the required receiver field of view (FOV), while stillbeing able to optimize the collection of the received signal. Forinstance, the scan rate is typically a few thousand degrees per second.For long-range targets, by the time the return signal is received thescanner mirror moves an appreciable amount. Consequently to enabledetection at both short range and long range a wider receiver FOV thanthe optimum is required. As disclosed herein, with the use of a smallmirror under computer control, it is possible to adjust the receiveralignment with respect to the transmitter as a function of the scan rateand range to the target. This allows a smaller optimized receiver fieldof view.

Beginning with FIG. 1, a block diagram is presented that depicts oneexample configuration of a topographic imaging lidar system 100 inaccordance with the disclosed embodiments. In this particularembodiment, the system 100 comprises a pulsed laser 1 with an attachedcollimator for producing a low-divergence beam. An example of such alaser is a fiber laser that can produce pulse energies of tens of microjoules in a 2 nanosecond wide pulse at pulse repetition frequencies ofhundreds of kHz and having a beam divergence less than milliradian. Thepulsed laser 1 is externally triggered from a pulse generator 2 andproduces a short optical pulse 20 which is directed onto a primaryoscillating scanner mirror 5 driven by a galvanometer scanner motor 12.An optical scanner comprising the primary oscillating scanner mirror 5,galvanometer scanner motor 12, and scanner drive electronics 9simultaneously deflects the outgoing transmit pulse 20 and the receivedreturn pulse(s) 22 from a target. Different scan patterns can be used(such as saw-tooth, sinusoidal, etc.) to obtain sample data points ofthe terrain that is being imaged.

A small fraction of the transmitted pulse energy is reflected by theterrain and then reflected by the primary oscillating scanner mirror 5onto an off-axis parabolic mirror 11 and onto a secondary scanner mirror13 before passing through a re-imaging module 15 containing lenses andspectral filter 14, and onto a detector 3, which produces an electricalsignal that is amplified by a receiver electronics 4. The TOF ismeasured by a time interval meter 6.

In the depicted embodiment, the system includes a position andorientation component 7 that includes global positioning system (GPS)positioning and inertial systems that are used for directgeo-referencing the location of the laser point on the terrain. Acontrol and data acquisition computer 10 (that includes electronics, oneor more processors, and memory components for storing and executinginstructions and non-volatile memory for storing data produced by thesystem) controls the operation of the system 100. For example, in oneembodiment, when the laser is fired, the control and data acquisitioncomputer 10 collects the measured data, which includes the TOF, scanangle, sensor position (e.g., latitude, longitude, height aboveellipsoid, etc.) and orientation (e.g., roll, pitch, heading, etc.). Thecontrol and data acquisition computer 10 is configured to time-stampseach piece of data and save it in a data storage unit such as, but notlimited to, solid state disk drives. In one embodiment, the system 100may include an externa, wired or wireless, interface such as an operatorinterface 8 that enables the system 100 to communicate with an externaldevice. For example, in one embodiment, the control and data acquisitioncomputer 10 may receive programming instructions and/or other data froma laptop computer for setting system parameters and monitoringperformance. In certain embodiments, the system 100 may be configured tocommunicate over one or more public or private networks (e.g., theInternet, an intranet, mobile data network, etc.) for sending orreceiving programming instructions and/or other data to and from thesystem 100. In some embodiments, the control and data acquisitioncomputer 10 is configured to also run the mission planning software,which provides the ability to plan the mission by graphically selectingthe survey area on an imported map, view the flight lines that areneeded to cover the survey area at the chosen flying height and monitorthe actual coverage and system status in real-time.

As shown in FIG. 1, in certain embodiments, the system includes asecondary scanner mirror 13 for implementing a method for reducing theprobability of detecting atmospheric backscatter. For instance, incurrent systems, after the transmitted laser pulse (depicted by transmitbeam 20) is reflected off the primary oscillating scanner mirror 5towards the target, if the range to the target is large and the scanrate high, the primary oscillating scanner mirror 5 rotates through anappreciable angle by the time the return pulse (depicted by receivedbeam 22) returns to the primary oscillating scanner mirror 5. Withoutthe secondary scanner mirror 13, the received light spot will move backand forth across the surface of the detector 3 as the primaryoscillating scanner mirror 5 oscillates back and forth. Thus, in currentsystems, where the size of the detector 3 determines the receiver FOV, arelatively large detector and hence a large receiver FOV is required.However, the result will be a less than optimum signal-to-noise ratio.

Thus, in accordance with the disclosed embodiments, the secondaryscanner mirror 13 is used to keep the received light spot on the centerof the detector 3, so a smaller receiver FOV can be used. In oneembodiment, the secondary scanner mirror 13 is synchronized to, anddriven at the same scan rate as the primary oscillating scanner mirror5. Because backscattered pulses from the atmosphere are more prevalentat close range, these undesirable backscattered pulses will fall on theedge of the detector 3 and hence will be greatly attenuated.

In one embodiment, to further reduce the potential for backscatteredatmospheric returns, the apparatus is configured to operate with areduced laser power, no more than what is needed for reliably obtainingrange measurements at the chosen flying height. In one embodiment, theminimum laser power needed for reliably obtaining range measurements atthe chosen flying height may be determined by executing an algorithmthat performs a lookup in a table containing minimum power laser levelsversus flying height. In some embodiments, this process is executedcontinuously in real time to adjust the laser power as the flying heightor terrain changes. The advantage of reducing the laser power to no morethan what is needed for reliably obtaining range measurements is toreduce the amplitude of unwanted backscattered signals from internalreflections and the atmosphere. Signals below the receiver detectionthreshold will not produce an output from the receiver and hence willnot cause a blind zone.

A second advantage of the disclosed embodiments is that for long-rangetargets where maximum laser power is required, the secondary scannermirror is positioned (rotated) with an offset with respect to theprimary scanner mirror, so that by the time the echo is received (afterthe TOF delay), the primary scanner has rotated so that the receiver FOVpointing direction will be in the optimum position to detect thelong-range echo, but will be misaligned for short-range atmosphericreturns. Thus, the probability of detecting atmospheric returns isreduced or eliminated.

As an example, suppose the primary oscillating scanner mirror 5 rotatesthe beams at a rate of 4000 degrees/second, the target range is 3500meters and the receiver FOV is 1 milliradian. The TOF for the 2-waytransit of the optical pulse will be approximately 23.4 microseconds.Thus, the scanner will have rotated the beam approximately 1.6 mrad bythe time the echo is received. Consequently, with the secondary scannermirror 13 operating as described above, the receiver FOV will bemisaligned by 1.6 mrad for close-range targets resulting in significantattenuation of return signals from close-range targets, thereby,reducing or eliminating blind zones caused by backscattered pulses fromthe atmosphere.

FIG. 2 shows the system 100 in operation. The system 100 is mounted inor on an airborne platform such as, but not limited to, an airplane 200.Using a pulsed laser, the system 100 generate a swath 202 produced by anacross-flight optical scanner and the forward motion of the aircraftresults in coverage along a track. As stated above, the system 100 usesGPS and IMU (inertial measurement unit) positioning for directgeo-referencing the location of the laser point on the terrain.

FIG. 3 is an example of a timing diagram showing a sequence 301 thatincludes three sequential laser trigger pulses and a correspondingsequence 302 of laser output pulses for the case where there is only oneoptical pulse in transit at any time. In the depicted embodiment,trigger pulse 311 produces laser output pulse 312, trigger pulse 314produces laser output pulse 315, and trigger pulse 317 produces outputpulse 318. Sequence 303 illustrates the received return pulses from thetarget resulting from the laser output pulses. For instance, echo pulse313 is the result of laser output pulse 312, echo pulse 316 is theresult of laser pulse 315, and echo pulse 319 is the result of laseroutput pulse 318. The corresponding blind zones (i.e., the time in whichthe system is blind to incoming signals) are shown as 320, 321 and 322.In a typical system, the width of the laser pulse is 2 or 3 nanoseconds,while the blind zone can extend for tens of nanoseconds, or more.

FIG. 4 is an example of a timing diagram showing a sequence 401depicting five sequential laser trigger pulses to the laser and acorresponding sequence 402 of laser output pulses for the case where thelaser is triggered before the echo (shown in sequence 403) from theprevious pulse has been received. Consequently, under this scenario,there are two optical pulses in the air at the same time. For instance,in the depicted embodiment, trigger pulse 411 produces laser outputpulse 412, trigger pulse 414 produces laser output pulse 415, triggerpulse 417 produces output pulse 418, trigger pulse 420 produces outputpulse 421, and trigger pulse 42 produces output pulse 423. Return pulse413 is the result of laser pulse 412, return pulse 416 is the result oflaser pulse 415, return pulse 419 is the result of laser pulse 418, etc.As stated above, because the laser is triggered before the return pulsefrom the previous pulse has been received, return pulse 413 is theresult of laser pulse 12 and not laser pulse 415, and return pulse 416is the result of laser output pulse 415, and not output pulse 418. Inthe depicted embodiment, the corresponding blind zones are shown as 433,434, 435, 436, and 437. If there are more pulses in transit at the sametime, the number of blind zones increases although the length of theblind zone is unchanged. As a result, the number of potentially wastedlaser shots is increased.

Referring now to FIG. 5, a flow chart illustrating one example of amethod of changing the laser firing time to avoid potential blind zonesis presented in accordance with the disclosed embodiments.

The laser is operated at a nominal PRF which is selected to provide therequired density of laser spots on the ground. In addition to the laserPRF, the spot density depends on the scan angle minimum to maximumrange, the scanner speed, the aircraft speed over ground and the flyingheight above ground. For each laser shot, the TOF will be the 2-wayrange to the target (there and back) times the speed of light. This TOF(which is accurately measured) determines when the return pulse isreceived from the target, relative to the time of emission of the pulsefrom the laser.

Since the laser is externally triggered and since the laser emissionoccurs at a fixed repeatable time after the laser trigger is applied,the time T between the optical outputs of successive laser shots (whereT is the reciprocal of the PRF at that instant) is known. Hence thetimes at which the blind zones occur are also known and can becontrolled by changing the laser trigger time. Specifically, on a shotto shot basis, T can be increased or decreased slightly to ensure that ablind zone is not coincident in time with the instant the return pulseis received. These changes in T are equivalent to small changes in thelaser PRF which do not significantly change the laser spot position onthe ground or the spot density.

As an example, if the laser is firing at a PRF of 200 kHz, there will beblind zones every 5 microseconds which may extend (for example) for 10nanoseconds. In the range to the target is 740 meters, the TOF will beapproximately 4.938 microseconds. In this example the echo will bereceived 62 nanoseconds before the blind zone. The preceding sequence ofTOF measurements may indicate that, for whatever reason (terrainvariation, aircraft position or orientation change, scan angle change,etc.), there is a high probability that the echo from the next lasershot will fall into the blind zone if an adjustment to the PRF is notmade. The software algorithm that controls the laser firing time mightthen shift the blind zone by reducing T by an amount of 100 nanoseconds(increase the PRF slightly) so that the echo now occurs after the blindzone, or the algorithm might increase T (reduce the PRF slightly) sothat the return pulse occurs well before the blind zone. These decisionsare based on knowledge from the preceding TOF measurements.

The process or algorithm is implemented as computer executableinstructions and is executed using one or more processor of thedisclosed systems. The process begins at step 501 by monitoring inreal-time the reported TOF (time of flight) of each laser output pulse.This is determined based on the time the laser output pulse is generatedto the time the corresponding return pulse is received by the system. Inthis embodiment, the sequence of TOF measurements are analyzed at step502, to predict if an upcoming TOF will be near a blind zone. In certainembodiments, the PRF may be adjusted based on user preference or basedon other parameters such as, but not limited to, flight data, type ofterrain, etc.

At step 503, if the process determines that an upcoming TOF will be neara blind zone, the PRF is adjusted (up or down), to avoid thisoccurrence. In one embodiment, if the return pulse comes less than 30nanoseconds before a blind zone, the next laser trigger pulse isadvanced by 50 nanoseconds (increase the PRF lightly) to make the blindzone occur before the expected time of arrival of the return pulse, orto delay the laser trigger pulse by 40 nanoseconds (decrease the PRFslightly) to make the blind zone occur an additional 40 nanosecondsafter the expected time of arrival of the return pulse.

If no adjustment to the PRF has been made to avoid a blind zone, theprocess at step 504 will make an adjustment to the PRF to make it closerto the initial PRF setting that was programmed for the survey mission toachieve the desired spot spacing and density.

In some embodiments, the adjustment may be performed by adding orsubtracting a constant value. Alternatively, in other embodiments, theadjustment may be performed by adding or subtracting a dynamic range ofvalues based on the determined upcoming TOF. In one embodiment, thesystem will make as minimum an adjustment as needed to cause theupcoming TOF to not be near a blind zone.

Although the depicted embodiment monitors TOF measurements in timesequence, in alternative embodiments, the process can be extended toextrapolate by referencing past laser shots as a function of scannerposition or calculated 3D position of the point.

Still in some embodiments, the system may be configured to dynamicallyadjust the minimum to maximum scan angle coverage so as to compensatefor the changing height of the aircraft and the changing elevation ofthe ground terrain to provide for a constant swath width and laser spotdistribution on the ground. In one embodiment, this is made possible bythe use of a programmable galvanometer-based scanner. The programmablegalvanometer-based scanner is configured to execute a swath-trackingalgorithm for the purpose of maintaining the desired laser spot densityon the ground. In systems without swath-tracking, the swath width is afunction of the programmed scan angle and flying height above ground.Consequently, in varying terrain heights, the spot density will not beconstant. In accordance with the disclosed embodiments, by dynamicallyadjusting the scanner parameters (e.g., the minimum to maximum scanangle coverage), the system is able to keep the spot density relativelyconstant as the terrain height changes during the survey mission.

Additionally, in certain embodiments, the system may also be configuredto compensate for changes in aircraft flying height and roll angle. Forexample, in one embodiment, using inputs from the GPS receiver andinertial measurement unit, the position and orientation component 7, asdescribed in FIG. 1, is configured to compute the aircraft roll angle inreal-time. Since the rotation axis of the primary oscillating scannermirror 5 is parallel to the aircraft's roll axis, the control and dataacquisition computer 10 is able to program the primary oscillatingscanner mirror 5 to compensate for the aircraft roll by offsetting theswath accordingly. Thus, this adjustment keeps the swath symmetricallycentered below the aircraft. The control and data acquisition computer10 also monitors the TOF data for every laser shot as measured by theTime Interval Meter 6, and calculates the slant range to the terrain.Using the calculated slant range to the terrain together with themeasured scan angle associated with each laser short, the systemcalculates the vertical elevation above the ground and a mean value isestimated, which is used to adjust the minimum to maximum scan anglecoverage (swath).

In another embodiment the median difference of the aircraft to grounddistance is used adjust the swath width.

In yet another embodiment, when mapping sloping terrain, the extent ofthe swath on the ground below the left side of the aircraft can be madedifferent from the extent of the swath on the ground below right side ofthe aircraft.

As an example of swath-tracking, suppose a planned survey calls for amaximum scan angle of 20 degrees at a flying height of 1000 meters aboveground, if the distance from the aircraft to the ground changes to 2000meters, either due to a change in terrain height or a change in theaircraft altitude, the system will dynamically reduce the maximum scanangle to 10 degrees in order to keep the swath width constant.

Thus, the disclosed embodiments provide system operators the benefit ofswath-tracking. An advantage of this additional feature includes nothaving to plan for wider than necessary swath widths to cover thepossibility of aircraft to ground distance changes, which results incost savings.

Accordingly, the disclosed embodiments provide one or more technicalsolutions to the problems associated with current airborne laserscanning systems. For example, in one embodiment, the disclosedembodiments provide an improved airborne laser scanning system thateliminates or reduces blind zones caused by unwanted scattered light(e.g., caused by reflections from internal optical components, a windowat the output of the system, a window in the aircraft through which thesystem operates, etc.) and backscattered light from the first few metersof the air below the aircraft. Additionally, as described above, thedisclosed embodiments provide an improved airborne laser scanning systemcapable of maintaining a constant swath width.

Although representative processes and articles have been described indetail herein, those skilled in the art will recognize that varioussubstitutions and modifications may be made without departing from thescope of what is described and defined by the appended claims. Forinstance, although the above description describes particular steps andfunctions being performed in a certain order and by particular modules,the features disclosed herein are not intended to be limited to anyparticular order or any particular implementation constraint. Forexample, one or more components may be added, reposition, removed,and/or combined in the embodiment described in FIG. 1 without departingfrom the scope of the disclosed embodiments. As another example, incertain embodiments, the process described in FIG. 5 may adjust the PRFwithout considering whether the PRF is at its nominal setting.

Therefore, it is to be understood that the invention is not to belimited to the specific embodiments disclosed. The scope of the claimsis intended to broadly cover the disclosed embodiments and any suchmodification or combinations as disclosed herein.

What is claimed is:
 1. A machine implemented method for reducing loss of data caused by blind zones in a laser scanning apparatus, the method comprising: dynamically monitoring a time of flight (TOF) of laser light pulse transmitted and received by the laser scanning apparatus; determining whether a potential collision of an outgoing laser light pulse and an incoming signal is likely to occur; and adjusting a pulse repetition frequency (PRF) in response to a determination that the potential collision of the outgoing laser light pulse and the incoming signal is likely to occur.
 2. The method of claim 1, wherein the potential collision occurs if a time of flight of the incoming signal is within a blind zone that occurs each time the laser is fired.
 3. The method of claim 1, wherein adjusting the timing of the outgoing laser light pulse comprises setting the pulse repetition frequency closer to an initial value if the setting was previously adjusted to avoid a potential collision.
 4. The method of claim 1, further comprising controlling special optical elements and a secondary scanner to prevent detection of unwanted echoes from the atmosphere.
 5. The method of claim 1, wherein the potential collision is based upon identifying a potential of an outgoing pulse and an incoming signal to be incident on a detector of the laser scanning apparatus synchronously.
 6. The method of claim 2, wherein the blind zone is caused by back-scatter light from an atmosphere or caused by back-scatter light from an optical surface.
 7. The method of claim 2, wherein the blind zone is caused by the back-scatter light from the atmosphere within 50 meters or less from an aircraft from which the outgoing laser light pulse is emitted.
 8. The method of claim 2, wherein the blind zone is caused by the back-scatter light from the atmosphere within 10 meters or less from an aircraft from which the outgoing laser light pulse is emitted.
 9. The method of claim 1, wherein the step of dynamically monitoring and the step of determining is performed on a shot-by-shot basis of the transmitted laser light pulse.
 10. The method of claim 1, wherein the step of dynamically monitoring, the step of determining and the step of adjusting the PRF are continuously performed in real-time.
 11. The method of claim 10, wherein the step of dynamically monitoring, the step of determining and the step of adjusting the PRF are performed on a shot-by-shot basis of the transmitted laser light pulse.
 12. The method of claim 1, wherein the step of adjusting the PRF adjusts the PRF to avoid a blind zone caused by back-scatter light from an atmosphere or caused by back-scatter light from an optical surface.
 13. The method of claim 1, wherein the potential collision is based upon identifying a potential of an outgoing pulse and an incoming signal to be incident on a detector of the laser scanning apparatus synchronously, and the step of adjusting the PRF adjusts the PRF to avoid a blind zone caused by back-scatter light from an atmosphere or caused by back-scatter light from an optical surface, the blind zone being the potential collision.
 14. The method of claim 1, further comprising continuously adjusting laser power as one of terrain height changes and aircraft altitude changes thereby reducing amplitude of back-scatter signals.
 15. The method of claim 1, further comprising positioning a secondary mirror with an offset with respect to a primary mirror to detect a long-range echo while being misaligned for short-range atmospheric returns.
 16. A laser scanning apparatus for reducing loss of data caused by blind zones, the laser scanning apparatus performing a method comprising the steps of: dynamically monitoring a time of flight (TOF) of laser light pulse transmitted and received by the laser scanning apparatus; determining whether a potential collision of an outgoing laser light pulse and an incoming signal is likely to occur, the collision caused by back-scatter or reflection; and adjusting a pulse repetition frequency (PRF) in response to a determination that the potential collision of the outgoing laser light pulse and the incoming signal is likely to occur. 